Interaction of tumor cells with infiltrating lymphocytes via CD70 and CD27 in clear cell renal cell carcinoma

CD70 upregulation by hypoxia-inducible factor and CD27+ lymphocyte tumor infiltration are associated with worse survival in von Hippel-Lindau gene (VHL) mutated clear cell renal cell carcinoma (ccRCC). CD70/CD27 interaction is accompanied by high soluble CD27 levels in the sera of ccRCC patients suggesting that soluble CD27 is a potential predictive tool for anti-CD70 therapy.

CD70 belongs to the tumor necrosis factor (TNF) ligand superfamily and is typically present on activated B and T lymphocytes, natural killer (NK) cells and mature dendritic cells (DCs). Immunohistochemical (IHC) analysis of several hundred renal cell carcinomas (RCCs) revealed elevated CD70 protein expression in about 80% of primary tumors and metastases of clear cell renal cell carcinoma (ccRCC). 1 A similar high expression frequency of CD70 was not reported for any other solid tumor types 2 thus making CD70 an auspicious therapeutic target for ccRCC. Studies elucidating the mechanism of CD70 upregulation and the role of CD70 interaction with its unique receptor CD27 in ccRCC are necessary to develop and optimize appropriate diagnostic and therapeutic tools, but these have been lacking so far.
The deregulation of the von Hippel-Lindau protein (pVHL)/hypoxia-inducible factor (HIF) axis is a hallmark of ccRCC. In this particular tumor subtype, we have identified that abundant CD70 expression is directly driven by HIF upregulation, which is a consequence of the loss of pVHL function by mutation of its gene 1 (Fig. 1, left). In addition to the molecular mechanism for abnormal CD70 expression, we also aimed to investigate tumor-infiltrating lymphocytes in the same tumor tissue samples by specifically focusing on the CD70 receptor, CD27, which is constitutively expressed on the surface of na€ ıve and activated T cells and memory B cells, as well as on a subset of NK cells. CD70/CD27 binding usually triggers a series of additional co-stimulatory signals, which results in the expansion and differentiation of memory and effector T cells as well as B cells. 3 The correct timing and intensity of CD70/CD27 signaling is mainly attributed to the restricted expression of CD70 on antigen-stimulated B and T lymphocytes and mature DCs. 3 Continuous CD27/CD70 interaction caused by constantly strong expression of CD70 in ccRCC may severely disturb the sensitive balance required for a proper regulation of the cellular immune response.
A detailed IHC analysis of ccRCC tissue sections has demonstrated that the lymphocyte infiltrates in CD70-expressing tumors indeed almost always contain CD27 C lymphocytes. As CD70 expression in pVHLdeficient primary ccRCC is retained in metastatic lesions, 1 it is conceivable that the persistent expression of CD70 leads to an enhanced attraction and infiltration of CD27 C lymphocytes during tumor progression. Interestingly, ccRCC with strong CD70 expression and tumor-infiltrating lymphocytes (TILs) correlated significantly with high nuclear differentiation grade and worse patient survival as compared to CD70-negative tumors without lymphocyte infiltration. We therefore conclude that durable interaction of CD70 and CD27 occurring between cancer cells and TILs may be an important step toward increasing the malignancy of a given tumor.
In transgenic mice, it has been shown that constitutive expression of CD70 on B cells or dendritic cells stimulates a rapid CD27-mediated phenotypic conversion of naїve T cells to effector cells, which culminates in the depletion of na€ ıve T cells thereby leading to lethal immunodeficiency. 4 In addition, fewer na€ ıve and central memory T cell subpopulations contrasting with increased numbers of effector memory cells have been detected in ccRCC tumors than among peripheral blood mononuclear cells (PBMCs) from the same patients. 5 This study also showed that CD70 combined with a T-cell receptor stimulus is sufficient to induce the differentiation of na€ ıve human T cells. This phenotypic conversion was also CD70-dose-dependent. 5 Consistent with these findings our results suggest that the co-existence of CD70 on tumor cells and CD27 C TILs fosters immune "exhaustion effects" by chronic CD70/ CD27 co-stimulation in ccRCC.
In our in vitro experiments co-culturing of PBMCs with RCC cells resulted in a CD70-dependent increase in soluble CD27 (sCD27) in the culture supernatant. A similar effect may explain why sCD27-concentrations in sera of patients with CD70-expressing ccRCC infiltrated by CD27 C lymphocytes have been observed to be significantly higher than those in sera of either patients with tumors negative for CD70 and CD27 or healthy probands. Huang and colleagues 6 showed that the extracellular domain of CD27 is released after lymphocyte activation and engagement of CD70. Release of the ectodomain of CD27 during the course of in vitro activation occurred with minimal cell death, indicating that sCD27 production resulted from shedding by metalloproteinases from the T-cell surface rather than from T-cell apoptosis. 6 Based on these data taken together with our own, we believe that CD70/CD27 interaction likely boosts the release of sCD27 into the sera of ccRCC patients.
As sera levels of CD70 are below the detection limit, the analysis of sCD27 may serve as a potential surrogate marker for CD70 expression. CD70 rather than CD27 is considered a main target for the treatment of ccRCC patients. CDX-1127 is a potent anti-CD27 agonist that induces robust antitumor responses primarily in patients with lymphoid malignancies expressing high levels of CD27. 7 Four anti-CD70 antibodydrug conjugates, MDX-1203, MDX-1411, SGN-75 and SGN-CD70A, have shown promising antitumor activity in CD70expressing RCC mouse xenograft models and some Phase I clinical trials for metastatic ccRCC are underway. 8,9 Provided the application of antibodydrug conjugates show significant response rates in further clinical trials, theragnostic approaches suitable for selecting those ccRCC patients who are likely to benefit from anti-CD70 therapy are required (Fig. 1). These include: i) the pathological determination of the clear cell subtype of RCC and detection of CD70 expression, as well as the presence of CD27 C tumor-infiltrating lymphocytes; ii) the treatment of ccRCC patients with an appropriate anti-CD70 drug; and iii) the analysis of sCD27 in patient sera to monitor and survey the clinical course of the disease. Although this scenario is conceivable, a number of retrospective and prospective studies will be required to confirm and validate these data.

Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.